1. Field of the Invention
This invention relates to articles consisting essentially of non-crosslinked polyvinyl alcohol. More particularly, this invention relates to elongate hollow articles in the form of tubing and fibers. This invention also relates to a method for preparing these articles from solutions of polyvinyl alcohol without chemically crosslinking the polymer. The prior art teaches the need for this type of crosslinking to prevent collapse of the article during or shortly after extrusion.
Background Information
Gels, fibers and other products prepared from solutions of polyvinyl alcohol in water or a mixture of water and a water-miscible organic solvent such as methanol, propanol, dimethyl sulfoxide and N,N-dimethylformamide are well known from numerous patents and other publications.
Typical of recently issued patents describing gels formed from aqueous solutions of polyvinyl alcohol is U.S. Pat. No. 4,663,358, which issued to Hyon et al on May 5, 1987. This patent teaches cooling to below room temperature a solution of polyvinyl alcohol in a mixture of water and a water-miscible organic solvent such as alcohols containing from 1 to 4 carbon atoms, glycols and dimethyl sulfoxide. Mixtures of water and dimethyl sulfoxide are preferred, and the water can constitute from 10 to 90 weight percent of the solvent mixture. The gel formed by cooling the solution is then immersed in flowing water to remove the organic solvent. Alternatively the initial gel is dried to remove substantially all of the solvent and then immersed in water to form the hydrogel.
Hydrogels prepared as described in the Hyon patent are transparent whereas gels formed using a solution of polyvinyl alcohol in either water or dimethyl sulfoxide as the only solvent are opaque.
Fibers formed from polyvinyl alcohol and various methods for preparing such fibers are discussed in a text by Ichiro Sakurada entitled "Polyvinyl Alcohol Fibers" (Marcel Decker, 1985). Most of the methods for preparing polyvinyl alcohol fibers that are described in this text use a solution of the polymer in either water or a mixture of water and a water-miscible organic solvent. The one notable exception was originally reported by H. M. Zwick and involves using a solvent or solvent mixture that causes phase separation into a solvent and a polymer-rich phase at a point between the spinneret and the takeup reel. The resultant fibers were washed to remove the solvent and then hot drawn using a draw ratio of between 2.6:1 and 8:1. Single solvents suggested for use in this method include benzenesulfonamide, caprolactam, and trimethylolpropane. Solvent mixtures include dimethyl sulfoxide/pentaerythritol and water in combination with urea or thiourea.
Various solvents for use in spinning polyvinyl alcohol fibers are taught in U.S. Pat. Nos. 4,698,194 and 4,603,083, which issued to Tanaka et al. on Oct. 6, 1987; U.S. Pat. Nos. 4,713,290 and 4,599,267, which issued to Kwon et al. on Dec. 15, 1987; and U.S. Pat. No. 4,765,937, hereinafter the '937 patent, which issued to Hyon et al. on Aug. 23, 1988.
The patents issued to Kwon teach preparing fibers from polyvinyl alcohol by extruding a heated polymer solution through at least one orifice and cooling it sufficiently rapidly to form a gel fiber, extracting the solvent used to initially dissolve the polymer with a volatile second solvent, and subsequently drying the fiber to form a "xerogel". The fiber is drawn in one or two stages at any point during or following cooling of the initially extruded fiber.
The fibers described in the aforementioned '937 patent exhibit tenacity values greater than 15 grams per denier. The degree of polymerization of the polymer used to prepare the fibers is preferably greater than 1700, and the polymer is dissolved in a mixture of water and a water-miscible organic solvent. Suitable organic solvents are disclosed in the aforementioned U.S. Pat. No. 4,663,358 to Hyon et al. directed to polyvinyl alcohol gels. Any of the conventional wet- or dry spinning methods can be used to form the fiber.
According to the teaching in the '937 patent, the temperature of the bath used to coagulate the spun fibers is preferably below -20 degrees C. After being dried the fibers are drawn at a temperature of from 140 to 220 degrees C. in air, an inert gas, silicone oil or polyethylene glycol using a draw ratio of from 20 to 200.
The use of boric acid or borate salts as crosslinking agents for polyvinyl alcohol fibers is taught in U.S. Pat. No. 4,612,157. The purpose of crosslinking is to improve the physical properties of the final fiber. The initial fibers are prepared by extruding a polyvinyl alcohol solution into an alkaline coagulating bath.
Japanese patent publication (Kokai) 02/112,407 describes a method for preparing fibers from polyvinyl alcohol and mixtures of polyvinyl alcohol with other fiber-forming polymers during which the fiber is repeatedly frozen and defrosted.
A method for preparing non-crosslinked solid core fibers from solutions of polyvinyl alcohol in non-aqueous mixtures of dimethyl sulfoxide and an alcohol containing from 1 to 4 carbon atoms is described in U.S. Pat. No. 4,851,168, issued to one of the present inventors, R. Kalinowski and G. Dahlbeck on Jul. 25, 1989.
Prior art methods for preparing hollow fibers and tubing from solution of polyvinyl alcohol always require a step during which the polymer is chemically crosslinked. Crosslinking maintains the integrity of tubing and avoids collapse of the hollow core in smaller diameter hollow fibers before the extruded profile is completely solidified in the coagulating bath.
Japanese patent publication 123,926, published in 1975, teaches forming the internal cavity in hollow fibers by the evolution of a gas such as carbon dioxide as the spinning dope used to form the fiber is extruded into a liquid medium. The gas can be generated by the reaction of a carbonate with an acidic medium used to coagulate the fibers.
A two-step "wet-dry" spinning process for hollow core crosslinked fibers similar to the one used to prepare conventional solid core PVA fibers is described in an article by V. Sakurada et al. that appeared in Polymer Journal, 19 (5) 501-503 (1987). Careful control of spinning conditions is required to achieve a continuous cavity within fibers that are extruded using a capillary spinneret.
In accordance with a method for preparing hollow fibers described in U.S. Pat. No. 4,073,733, which issued to K. Yamaguchi et al. on Feb. 14, 1978, a solution of PVA prepared by dissolving the polymer in a polyethylene glycol at 100.degree. C. is extruded into an aqueous coagulation bath containing sodium hydroxide and sodium sulfate. The relative weak fibers are then immersed in a hot aqueous solution of glutaraldehyde, sulfuric acid and sodium sulfate to convert the polymer to a crosslinked material, following which the polyethylene glycol is extracted from the fibers.
Crosslinking of PVA hollow fibers using formaldehyde is taught by C. Cheng, Y. Chang, M. Chen and A. Tobalski in the Journal of Applied Polymer Science 17, 789 (1973). The resultant fibers exhibited high levels of water permeability, mechanical properties and salt rejection.
Methods for preparing hollow fibers prepared from crosslinked polyvinyl alcohol are also described in the following patents and other publications:
J. Chromotogr. 368(2) 391-4 (1986) PA0 Japanese patent publication 61/42301, published Feb. 28, 1986 PA0 Japanese patent publication 58/40102, published Mar. 9, 1983 PA0 Japanese patent publication 75/113809, published Jul. 15, 1982 PA0 Japanese patent publication 56/10533, published Feb. 5, 1981 PA0 Japanese patent publication 56/11931, published Feb. 5, 1981 PA0 Japanese patent publication 52/21422, published Feb. 18, 1977 PA0 1) preparing a 2 to 30 weight percent solution of polyvinyl alcohol in a liquid mixture consisting essentially of dimethyl sulfoxide and from 10 to about 30 percent, based on the weight of said mixture, of a liquid that is miscible with dimethyl sulfoxide and is selected from the group consisting of water and alcohols containing from 1 to about 4 carbon atoms, where the number average molecular weight of said polyvinyl alcohol is at least 44,000; PA0 2) extruding the resultant polyvinyl alcohol solution in the form of said article into a coagulation bath concurrently with extrusion of a first non-solvent for polyvinyl alcohol, where said coagulation bath consists essentially of a second non-solvent for said polyvinyl alcohol and is maintained at a temperature no higher than -10.degree. C., said solution and said first non-solvent are extruded through a die having a perimeter and an outer orifice that is in close proximity to said coagulation bath and a hollow mandrel positioned within said die to provide an elongated article having a central core; the outer orifice defining the outer contour of said article and the outer surface of the mandrel defining the hollow central core of said article, the hollow mandrel having a first passage therethrough culminating in an inner orifice which is in communication with the hollow central core of the extruded article, and where said PVA solution is extruded through a second passage defined by the perimeter of the die and the outer surface of the mandrel, concurrently with extrusion of said first non-solvent through said first passage at a rate sufficient to maintain the contour of said central core during coagulation of said article; and PA0 3) removing said article from said coagulation bath and displacing at least the dimethyl sulfoxide portion of the liquid mixture retained in said article with water or a liquid alcohol.
Hollow fibers prepared from a blend of formaldehyde-modified Nylon 66 and polyvinyl alcohol are described in British patent No. 1,577,547, which issued on Oct. 22, 1980.
One disadvantage of articles formed from crosslinked polyvinyl alcohol (PVA) is the inability of these articles to absorb aqueous liquid media and swell to at least twice their original dimensions while developing certain desirable surface properties such as lubricity. These properties together with their biocompatibility make non-crosslinked polyvinyl alcohol hydrogels uniquely suited for certain medical end use applications described in detail elsewhere in this specification.
Hollow fibers have been prepared from uncrosslinked blends of a polyvinyl alcohols and other polymers. U.S. Pat. No. 4,039,499 describes the preparation of such fibers from blends of a polyvinyl alcohol and an N-alkoxyalkyl polyamide. German patent No. 2,615,954, which issued on Nov. 18, 1976 teaches preparing hollow non-crosslinked fibers from blends of PVA and a polyethylene glycol.
The biocompatibility and antithrombic properties of polyvinyl alcohol make it a desirable material for use in fabricating or coating articles such as catheters that are implanted or passed through the bodies of animals, particularly humans. Coating catheters with PVA hydrogels to reduce blood clotting is reported by M. Okada et al. in Japanese Patent Publication No. 01/115,362, which issued during May of 1989. The catheters were formed from polyether/polyurethane copolymers.
One objective of the present invention is to provide a method for preparing tubing and hollow fibers from polyvinyl alcohol. A second objective is to provide small diameter tubing formed from a non-crosslinked polyvinyl alcohol in the absence of other polymers.